The invention relates to methods and apparatus for locating bone cuts on the medial and lateral femoral condyles to form seating surfaces for a femoral knee prosthesis, and to coordinate tibial and patellar resection and replacement with femoral resection and replacement.
The invention further relates to a tool for locating said cuts.
Over the years, the concepts of designs for the total knee arthroplasty have evolved to the point where with few exceptions, most are quite comparable in the design of femoral, tibial and patellar prostheses.
Major discrepancies and problems encountered are caused by physician error and failure to understand the principles of more complex alignment or ligament problems to be corrected at surgery. With the more complex alignment or xe2x80x9croutinexe2x80x9d degenerative knee, the major differences are the ease and consistency of instrumentation for alignment and proper bone cuts allowing proper ligament balance. This allows satisfactory motion and stability post operatively.
The distal femoral cuts must be placed to provide the knee prosthesis with a proper flexion and extension gap, proper varus-valgus alignment, proper patellofemoral relationship and proper rotation. It is customary to use an intramedullary rod placed in a retrograde fashion between the medial and lateral femoral condyles just anterior to the intercondylar notch to establish a single point of reference for subsequent bone cuts. A major problem is in the instrumentation to indicate the location of the femoral cuts which relies upon the xe2x80x9cexperiencexe2x80x9d or xe2x80x9ceyeballingxe2x80x9d of the surgeon. Over the years, two basic instrument system designs have become popular.
In one design (anterior referencing), the total knee alignment system takes its point of reference from a centrally placed rod and careful attention is given to the patellofemoral joint by using an anteriorly placed feeler gage. The distal femoral cut is consistent with the thickness of the prosthesis.
This instrument system operates on the principle of anatomic anterior and distal femoral cuts to allow proper ligament balancing and stability in extension as well as consistent patellofemoral placement on the anterior surface. The femur is not notched, and the anterior surface of the femoral prosthesis not elevated above the anterior surface of the femur. Notching the femur may cause a decrease in strength of the distal femur. Elevation of the anterior surface of the prosthesis will cause extremely high patellofemoral pressures in a joint that seems to be prone to a high rate of post-operative failure.
By establishing the anterior femoral cut as the benchmark or datum starting point, however, the anterior referencing instruments result in the installation of a knee prosthesis which sacrifices consistent stability in flexion due to the formation of a posterior femoral condylar cut that may leave the posterior space either too wide or too narrow. This can cause instability in flexion, or restrict flexion and cause increased wear.
The second type of instrument design (posterior referencing) is based on the concept that the flexion and extension stability are more important and the patellofemoral joint is of secondary importance. This system also uses an intramedullary rod for referencing. Although I consider all three joints as xe2x80x9cimportantxe2x80x9d, when a compromise must be made, the posterior referencing systems compromise the patellofemoral joint while the anterior reference systems sacrifice stability in flexion (the posterior tibial femoral joint). Both systems allegedly equally address the distal tibial-femoral space. Neither consistently addresses the distal rotation of the femoral component.
Neither system tries to preserve the joint line at or near an xe2x80x9canatomicxe2x80x9d level. By elevating the jointline, the patella is distalized. The femur is also shortened. Since the arthritic knee often has a loss of cartilage, there may be a patella infera of 2-3 mm initially. Elevating the distal femoral resection beyond this will:
1) Further alter the patellofemoral relationship.
2) Change the isometric and rotational balance of the MCL and the LCL.
3) Shorten the femur in flexion and may cause increased roll back, anterior lift off, and increased posteromedial wear.
4) Elevate the level of tibial resection necessitating a major amount of posterior femoral resection to achieve a satisfactory flexion space.
When performing a unicompartmental knee replacement, it is imperative to maintain the jointline. As a consequence, it is desirable to maintain a full range of motion.
An object of the invention is to provide methods and apparatus for locating bone cuts on the medial and lateral femoral condyles to form seating surfaces for a femoral knee prosthesis, and to coordinate tibial and patellar resection and replacement with femoral resection and replacement which reliably and anatomically provide:
1. Consistent distal tibio-femoral stability.
2. Consistent distal femoral rotation.
3. Consistent placement of the anterior cut flush with the anterior surface of the femoral cortex, i.e., without notching or elevation.
4. Consistent placement of the posterior femoral cut such that the distal and posterior cuts are equal (when indicated) allowing for satisfactory extension and flexion stability and motion.
The method and apparatus of the invention contemplate placement of the anatomic joint line which, in extreme cases, varies up to the difference between the anterior-posterior A-P internal measurements of the size prostheses. Based on my knowledge of total knee replacement, personal experience with numerous routine total knee replacements, numerous more complicated cases consisting of knees with flexion deformities and revision surgery, a maximum of a few mm proximal or distal displacement of the joint line is considerably less harmful than:
1. A lax flexion gap;
2. Sloping the proximal tibial cut to accommodate for an inconsistent posterior femoral condylar cut;
3. Significantly notching the femur anteriorly;
4. Raising the anterior flanges of the prosthesis and thus the patellofemoral joint;
5. Not allowing full extension;
6. Raising the joint line;
7. Tightness in flexion;
8. Malrotation; and
9. Patient pain.
With an understanding of the measurements involved in total knee replacement, a new instrument system and methodology has been developed to allow flexion 120-130 degrees; to perform less soft tissue releasing; and decrease surgical time. Starting with a xe2x80x9cnormalxe2x80x9d knee, the goal should be to maintain the anatomic landmarks as close to normal as possible. Then, if deformities are present, the procedure can be modified to accommodate the situation.
In accordance with the invention, a method is provided for forming planar cuts on the medial and lateral condyles of the femur to form seating surfaces to receive a femoral knee prosthesis, comprising:
determining a prospective planar cut at the posterior of the condyles of the femur at which the distance between the anterior surface of the femoral cortex and the prospective planar cuts is substantially equal to the interior dimension of a knee prosthesis to be fitted on said femur at the anterior surface and the cut planar surface,
determining the thickness of the posterior lateral or medial condyle which will be resected by said prospective planar cut,
cutting the distal ends of the condyles along a plane at which the maximum thickness of resection of the more prominent condyle at said distal end is substantially equal to the thickness determined to be resected at the posterior medial or lateral condyle by said prospective planar cut, and
cutting the condyles along a plane substantially flush with the anterior surface of the femoral cortex, and along said prospective planar cut.
The method further contemplates loosely placing a longitudinal intramedullary rod in the femur such that an end of the rod projects from the femur, mounting a tool on the projecting end of the rod, establishing, by said tool, an angular position of said prospective planar cut along a plane rotated at an angle of between 0 and 15xc2x0 with respect to a tangential plane at the posterior of the lateral and medial condyles about an axis located in said tangential plane.
In further accordance with the method, the tool is rotated with said rod through said angle and a datum or benchmark is established by the rotated rod or by pins installed in the condyles on the basis of the rotated position of the tool. A cutting guide can be mounted on said tool, to enable the distal end of the condyles to be cut along said plane. Thereafter, the tool is removed and a second A-P cutting guide is mounted on the selected benchmark, i.e., the rod or the pins and the posterior and anterior cuts are made. The axis about which the plane of the prospective cut is rotated is located in said tangential plane at the posterior surfaces of the medial and lateral condyles and can be located at either of the condyles or at any location therebetween. It is a feature of the invention that the tool may remain on the rod both for the measurements and for the cutting of the distal end of the femur.
The invention also contemplates that the cutting guide supports a means which enables the cutting guide to be secured to the condyles during the cutting of the distal ends of the condyles.
The invention further contemplates an apparatus for forming planar resections on the medial and lateral condyles of a femur to form seating surfaces to receive a femoral prosthesis and to properly articulate with a tibial and patellar prosthesis comprising:
a caliper feeler and measurement plate to measure for the size of the femoral prosthesis to be received, said caliper feeler and measurement plate adapted to determine a first distance between an anterior surface of the femoral cortex and a plane tangent to a posterior surface of the medial and lateral condyles of a femur, the caliper feeler referencing the anterior surface of the femoral cortex and the measurement plate referencing the plane tangent to the posterior surface of the medial and lateral condyles;
a graduated scale to compare the first distance to at least two standard femoral prosthesis sizes and to determine the smaller of the at least two standard femoral prosthesis sizes;
a graduated scale to measure a second distance between the first distance and the size of the smaller standard femoral prosthesis size, so that a thickness or thicknesses can be measured to be resected at the posterior surface of the medial and lateral condyles of the femur by adding the average thickness of the posterior condyles of the smaller standard femoral prosthesis and the second distance;
a tool to resect the medial and lateral condyles along a plane at the anterior surfaces thereof flush with the anterior surface of the femoral cortex; and
a tool to resect distal ends of the medial and lateral condyles at a resected thickness equal to the average thickness of the distal condyles of the smaller standard femoral prosthesis plus the second distance.
The apparatus further contemplates a tool to resect the measured thickness at the posterior surface of the medial and lateral condyles of the femur.
The invention also contemplates a method for forming planar resections on the medial and lateral condyles of a femur to form seating surfaces to receive a femoral prosthesis and to properly articulate with a tibial and patellar prosthesis comprises:
measuring for the size of the femoral prosthesis to be received by determining a first distance between an anterior surface of the femoral cortex and a plane tangent to a posterior surface of the medial and lateral condyles of a femur;
using a graduated scale to compare the first distance to at least two standard femoral prosthesis sizes;
measuring a second distance between the first distance and the size of the smaller standard femoral prosthesis size; and
measuring a thickness or thicknesses to be resected at the posterior surface of the medial and lateral condyles of the femur, the thickness being equal to the average thickness of the posterior condyles of the smaller standard femoral prosthesis plus the second distance.
The method further contemplates the steps of resecting the medial and lateral condyles along a plane at the anterior surfaces thereof substantially flush with the anterior surface of the femoral cortex; and
measuring a thickness or thicknesses to be resected at the distal ends of the medial and lateral condyles, the thickness being equal to the average thickness of the distal surface of the smaller standard femoral prosthesis plus the second distance, and resecting the distal ends of the medial and lateral condyles at the measured thickness.
Other features and advantages of the present invention will become apparent from the following description of the invention which refers to the accompanying drawings.